Push fit or solvent weld waste pipe: which joint

The basin is off the brackets, the trap is in the bucket, and the stub in the wall still says 40 mm. The pipe you bought says 40 mm as well. You line the spigot up with the socket, give it a twist, and it stops after a few millimetres. Washing-up liquid does not move it.

You are choosing push fit or solvent weld waste pipe, and the two systems share a nominal size while the actual tubes differ. JDP and B&Q both say not to push one into the other. Match the pipe already in the wall, or fit a conversion piece that names both systems.

Peter F. has not glued sample joints in a lab. Hold times below come from Kayflow's installation sheet. The 25 mm pipe note comes from the amazon.co.uk listing in the PVC pipe comparison. A qualifying purchase from a link to amazon.co.uk can earn a commission at no extra cost to you.

In short

Push-fit waste is polypropylene and comes apart. Solvent weld is often ABS and stays put, at the same nominal sizes, and the spigot of one will not enter the socket of the other. Use a conversion fitting if both are already in the house, with 32 mm on a basin and 40 mm on a sink, shower, or bath.

Two systems, one nominal size

The number moulded on the fitting is a name, not the outside diameter a tape will read. A 40 mm push-fit pipe and a 40 mm solvent socket are both sold as 40 mm, and they are not the same tube. B&Q's waste guide says the two systems are slightly different sizes, and EasyMerchant says the same about adaptors: same nominal size, different actual size. That is why a push-fit end still stops in the old solvent socket under the sink.

JDP describes push-fit waste as polypropylene with a rubber ring. Solvent weld is often ABS, and the joint is permanent once the cement has grabbed. Polypropylene is not the plastic that cement welds, so a stuck spigot needs the right fitting, not a smear from the tin.

Read the pipe you took off before you buy. A rubber ring means push-fit, and a plain glued socket means solvent weld. Stay on that system for every bend and length. The PVC pipe comparison sets a stated outside diameter next to the job on the tin. The top pick there states 25 mm outside, solvent-weld, 1 m, at about £10.20, with 4.2 from 17 UK reviews. It is the wrong pipe for a 32 mm basin or a 40 mm sink.

Push-fit when you may need to undo it

Choose push-fit when you may have to take the run apart. The trap rarely lines up with the wall boss on the first dry fit. With a ring joint you pull the bend off, turn it, and push it home again. B&Q calls that the easier system to undo, and JDP calls the joint demountable. Buy push-fit pipe and push-fit fittings only.

Have any conversion fitting on the bench before you are under the basin. A push-fit bag will not enter an old solvent socket. The ring seals only when the pipe is fully home and the burr is off the cut, so the spigot does not roll the ring out. If your depth mark sits proud of the socket mouth, the joint is not home.

The limit is a pipe that gets knocked. Under a kitchen sink the pedal bin and the washing-up bowl share the cupboard with the waste. A rubber ring holds by compression, and a joint that keeps getting struck works loose. Cement will not turn that polypropylene pipe into ABS.

Solvent weld when the joint might be knocked

Choose solvent weld when the joint might be knocked and you want it to stay. JDP treats the weld as permanent, and B&Q calls it the stiffer joint if the pipe might be struck. A kitchen waste that crosses the cupboard to a side-wall boss is the usual place. Once the cement has set, the bend stays put when you drag the bin out.

Kayflow's installation sheet says you hold the cemented joint for 10 to 20 seconds so it cannot push back out, and that you can handle it after about five minutes. Those times belong to that sheet. Follow the tin in your hand for when water may run. The joint should be dry. You twist the pipe home as you push, and you hold it still, with the window open and the lid on between fittings.

You cannot undo a solvent socket. A bend that points at the pedestal has to be cut out and replaced on a clean, square end, so buy a spare bend before you open the tin. Solvent-weld waste is often ABS, and some brands specify their own cement. If the old pipe is polypropylene push-fit, leave the cement in the bag.

How to join them if the house already has both

The trap you can see is often push-fit, and the boss in the wall is still a solvent socket. Do not file the spigot until it enters. Fit a conversion piece made for push-fit to solvent weld at the same nominal size, 32 mm to 32 mm or 40 mm to 40 mm, and check that the packet names both systems.

A reducer from 32 mm solvent to 40 mm solvent will not marry a push-fit pipe to a solvent socket. Dry-fit the adaptor. It should go home by hand. If it needs a hammer, it is the wrong fitting. Keep the adaptor where you can reach it. On a basin, sit it between the trap and the wall and clip it. A chain of adaptors means you should strip the run back to one system.

Replacing the branch on the existing boss, at the same nominal size, is the job this choice is for. A new hole in the soil stack, or a branch that a bath and a basin share, is when you book a plumber. Once the pipe is a drain, Approved Document H applies. The fall on that buried run is a separate decision in the underground drainage pipe comparison.

Sizes for a basin, a sink, and a bath

Kayflow's guide puts a basin on 32 mm, and a sink, shower, or bath on 40 mm. A run that groups more than one appliance steps up to 50 mm. Which waste pipe for a kitchen sink is 40 mm, in the system already in the wall. The copper pipe comparison is the supply to the tap, which is a different pipe from the waste.

Match the number on the old pipe. A 32 mm push-fit basin waste is not replaced by a 40 mm solvent length, and a longer run that will also take a shower needs a step-up from the same system. Take that printed number to the merchant with an offcut if the stamp is worn.

You do need clips. An unclipped waste sags and holds sludge. Kayflow cites spacing to BS EN 12056, so use the spacing printed for your pipe, with a clip beside any fitting that would lever the joint. Clips do not fix a mismatch between the two systems.

Outside, either system can carry a short waste if the joint matches. Drainage Pipe UK says to paint the pipe if it is used outside. Paint does not make the outside diameters agree. If the run drops into the ground, it has stopped being a basin waste. How a PVC joint is made sits with how to join PVC pipe. Whether a short black stick is really 32 mm or 40 mm solvent waste is the check on solvent weld waste pipe.